Infrared flaw detector method and apparatus



Sept. MJSS La. MAURO-" i, 3,2%,593 ffii Ixmml'mw DETECTOR mami Aap Filed Aug-*15's .lss2- .v 3 sheartmn, 3

INFRARED FI 5 y AND APARATUS .lack A. Mauro, itsield, Mams., assigner to Electric Company, n corporation of New ork Filed Aug. 16, i962, Ser. No. 217,317 14 Claims (Cl. 25W-83.5)

My invention relates to a method and apparatus for detecting tlaws within heat conductive mediums and, in

particular, to a method and apparatus for. detecting minute subsurface and interlaminar voids and other. flaws in com@ posite metallic members wherein the detection is accomplished by a noncontacting and nondcstructive process. Melding brazing, cladding and electroplating are several methods employed for joining metals in the fabrication of aircraft, space vehicles and engines. The environmental conditions under which thefinal components are subjected demand strength and reliability of the fused parts. Under high temperature operational conditions, minute voids in the' subsurface region of the metallic component, particularly in the intcrlarninar region that joins composite metallic members, can expand suiciently to rupture the joint. Gaseous accumulations in the interlaminar spaces due to faulty fusion of metals can result in chemical interactions that impair the performance reliability, strength and durability of the component. In like manner, variations in the thickness of a heat conductive coating on a base heat conductive material may result in significant changes in operation of the component. A

' which allects the r'low of radiant energy through, or recoating thickness.

' Methods and instruments presently used to detect aws in solid materials, and particularly -in composite memv bers comprising a base portion and an overlying or clad portion, employ numerous devices and materials including evaporgraphs, phosphors, thermal paints, thermal dyes,

ultrasonics, and imageconvcrter Satisfactory operular shape -or geometrical configuration of the material size of the probe or sensing device. In the case of voids,/

though suchinstruments may provide gross and qualitaj'l r tive data, they do not provide preciseand .accurate quanL ti'tative data concerning subsurface voids smaller than 0.070 inch diameter. Further, conventional methods and' order of 1%000 inch in the thickness of a metallic coating -by a noncontacting and nondestructive process. Since presently used methods and instruments cannot be employed to detect subsurface voids smaller than 0.670 inch diameter or coating thickness variations of 0.010 vinch with a high degree of accuracy, a need exists.Y for solving this problern'by providing a new methodf and apparatus for aw detection. y Therefore, one of the principal tion is to develop a new and improved method and zippaiatus r'or detecting extremely small subsurface voids in amaterial. ,im Another important object of my invention is to develop minute subsurface voidswhereby no physical contact is required between the material being examined and the istics of the material being examined are not altered.

A still further important object of my invention is to' Q develop a new and improved method and apparatus for ..":V" 4 i j' ,aw oaracro'n Marilou VGenerall flaw as referred to herein is defined as any inhrnnogeneity` ection from, a material, thus it includes variations in t 35/ FIGURE 6 illustrates a modification of rnyri-nvcntiontion of these instruments is usually limited by the partic- I being examined, character and depth of the flaw, 4and the] instruments cannot accurately detect variations in the' ll objects of my invern a new and improved method and apparatus for detecting ant object of invention isv tor. develop a new and improved method and apparatus for detecting minute subsurface voids whereby the character- 3,295,663', i Patented sept. 14, teas detecting small variationsin the thickness ora heat con-1 ductive coating on a' base material. Briefly stated, and in accordance with one aspect of my invention in meeting the objects enumerated above, `l provide an apparatus that applies energy in a nondes-v tructive manner/to the surface of a material being n Spectec', thereby, establishing a thermal gradient on the surface thereof/ in accordance with occurrence of voids or other flaws/within the material. A detector compo uen: of the apparatus senses the thermal gradient which may be recorded whetefrem the location and size ofovoidx or flaws may' be determined.

The features of my invention which I desire to protect herein-fare pointed out with particular-ity in the ap pended claims. The invention itself, however, both as to its organization and method of operation, together with further -objects and advantages thereof, may best be 21uderstoodl by reference to the following description taken in connection with the accompanying drawing, wherein like parts in each of the several figures are identified by the same reference character, and wherein: FIGURE 1 illustrates a general arrangement of a flaw detector constructed in accordance with my inventinand employing a conventional thermal radiation detector; 25 FlGURE 2 is a detailed view of arotating chopper mirror shown in FIGURE l; FIGURE 3 illustrates a modification of my invention indicating another means for applying energy to the sur# face of a material being inspected; Y /FIGURE 4 illustrates a test specimen with selectively the same side of the material being tested;and FIGURE d is agraphical result of tho-use of my invention with the specimen of FIGURE 4, plotting thermal ,gradient versus time. Y

are

wherein a variation in coating thickness may be detected and the heating and detection means are on opposite sides of the material being inspected; FIGURE 7 is a modification of the invention ofFIG- URE 6 wherein the heating and detection means are on the same side of the material being tested; and FIGURE 8 illustrates a modication o my aw detector invention wherein a heating and detection means are on opposite sides of the material being inspected.

Referring particularly to the apparatus illustrated in FIGURE l, there is shown a heat conductive material 1 to be inspectedthat moves in the direction of arrow 2 in noncoutacting relationship with respect to a convert tional thermal radiation detector indicated as a whole by numeral 3. The particular conventional thermal radiation detector shown in FIGURE l will be described in vdetail to illustrate a method whereby such devices may -hc employed to detect subsurface voids and other aws in materials. An energy source for generating` heat within material 1 comprises an essential element of the flaw detecting apparatus. The energy may be supplied by radiation, induction or conduction. A radiant energy source indicated as a whole by numeral 4 and illustrated schematically as a heated filament 5 and convex lens 6 with. in a paraboloidal or ellipsoidal body or frame 7, impingcs its emitted rays 8 upon a surface of the material 1 at -ocal point 9. The energy of rays 3, being focussed at 1 point 9, thus generntesa predetermined amount of heat Dwhich is absorbed by material I. This heat is preferably vsupplied at a constant rate and material 1 moves at a constant rate thercbycifccting a constant risc intemperie ture of the surface of material 1 in the absence of any voids or flaws therein. Howevcnwithin those portions of material 1 wherein a subsurface -void or flaw cxists, change in the surfaceconstant temperaturerischecinafter referrcltg as a 4thermal gradient, occurs.' The oo- :Londons casion of a thermal gradient maybe explained lnthe following manner. Airis a relatively poor heut" confthe emitted radiant energy therefrom may be -eontrolleclf ductivc material, and a void or other llaw iocatcdwithin by controlling the temperature thereof. Thermocouple.- the sub-surface of a relatively good heatconductive ma- 26 and its associated leads 27 maybe connected to afi terial may be considered to be a minute air pocket. Thev temperature indicating device (not shown) to monitor the energy, absorbs radiantenergy but rellccts no energy,` thus application of heat to the surface of a good heat eonductemperature or' black body 25. Hereafter the radiation tive material having a minute void within its subsurface `or energy emitted from black body 25 will be referred l effects a change in the temperature distribution therein to as the reerenceenergy. The reference energy is tid- V since the normal distribution of isothermal lines within iustcd to be equal to the energy of focussed rays 15 and the good heat conductive material is disrupted by the l3 0 16 in the absence of any void or llaw within material Lf presence of a poor heat conductive region. This change -Ths reference energy is either reflected hy mirror 19 to in the temperature distribution results in what may be bolometcr 18 (as shown) or passes directly to bolomcter i described as a hot spot at the surface directly above the 17 when mirror .19 is in a nonihterfcring mode. Thus subsurface void'in the case where heat is applied to. the bolometcrs 17 and 18 receive alternate pulses of focussed surface from the same side of the material as the thermal 15 energy rays 15, 16 and alternate pulses of reference en- Y radiation detector is disposed in relation thereto. ln ergy troni black body 25 through the action of rotating' like manner, a cold spot is sensed by the detector when Choppcrmirror 19 in a sequence lwhereby the emitted '5 the heat source is positioned on the opposite side of the radiation or temperature of th: surface'of material 1 is material from the detector. compared with the black body radiation or tcrliperciturerl l Thus, a thermal gradient or change in the surface tem- 20 The outputs of bolometers 17 and 18 are electrical pulses' f. perature indicatesthepresence of a void or other aw which pass through a preamplifier stage and are then arn-L- Mwithiri thesubsurfa of material l, the thermal difference plilied by amplifiers 28 and 29 and the outputs thereof at thc surface being a function ofthe size of the void and areconnectcd to integrator 30. The bolcrnetcrs and their I the heat conductive characteristics of material 1. associated amplicrs are thus connected in a differential i i in general, the detectability of thermal differences deg5 arrangement whereby in the absence of a void or flaw pends upon the sensitivity of the detector material emwithin material 1, the output of each amplier is'equal poyed and the' radiation temperature of the material and the output of integrator 30 is zero, Gnly in the 1 being inspected, Radiant nei-gy in the infrared ponicn presence of a void or tlaw will integrator 3i) register an f` ofthe radiant energy spectrum is preferably employed al Output as a result of the dilerent emitted radiation dethough it is' to be understood that other portions of the 30 tccted as chopper mirror 19 rotates and material 1 moves spectrum may also be utilized, the only requirement being lhcl'eby- The OUPU 0f lmegmol' 30 S COHHEC t0 a that the radiant energy emitted from the material 1 :-.urbrush recorder 31 which provides a visual recording of'l face and indicated as emitted rays and 11, he of sufthe results. The amplifier- 23 and 29, integrator 30,'. cieut intensity tobe easily sensed by thermal radiation and recorder 31 may be any conventional electrical com- -r detectar s. A These rays 1o and 11 am illustrated as emit- 35 poncnts of this type wcllkuown in the art. The trace.' ting from point l2 on the material v1 surface due to the frOm brush recorder 31 forms a permanent record of angle of incidence of rays S, and are characteristic of the thermal contours or isotherms, and relative to the straightgnduccd mem-m1 gradient hat indicates me presen@ Of line, the reference temperature level or axis-of abscissasv" 1 void 13 located in the subsurface of the material. lAri 0 lh@ graph 0f FIGURE 5, ndes the DCdlOrl and t.. optical system is employed to image the small arca being 40 relative size of the voids as a function ol the temperature l examined and illustrated as point 12, upon the actual gradients. 5, sensing element of detector 3 and, as illustrated, permits- FIGURE 1 alSO UUSlfilleS l mOdfCU-GYI 0f N: mea 'entrance only of emitted rays l0 and 11 from the surface Jl' applying QUCVEY l0 ille Sufflce 0f mefal 1- This of material l into the sensing portion of the detector,4 -modicaion illustrates an energy source that generatesff The optical system may comprise a single lens or a multit5 heating within material 1 by induction.- An induction lens system. A Cass'egrain mirror telescope which is free heating coil 32 is wrapped around material 1 and com' of chromatic aberration and' absorption isillustrated by1 nected to a suitable source of power at terminals 33 to Way of example in FIGURE l although other optical 'generate an adequate surface temperature.' 1 vsystems may also be employed. "Rays 10 and 11 are FIGURE 3 illustrates another modification of-thej v shown as rays 14 within this system and finally are emitted 50 means for applying energy tothe surface of material 1- therefrom as focussed rays 15and 16. being inspected and moving in the direction of arrow 2..

The sensing means employed for detecting a thermal vThislrlodilication illustrates an energy source-that gengradient on the surface of material 1 is positioned in thecrates healing within material 1 by conduction. Tube nal image p'ane of the optical system and comprises two 34 has at one end flexible tubing 35' and supply control conventional thermal sensitive detectors known as'lhermis` 55 valve 36, andagas flame 37 at the other end. Valve 36 .tor bolometers- It is to be understood that any infrared F-.corltrols 'ii'gas supply and therefore the size of the flame detector may be utilized. Howcver, where thermistor 37. The flame 37 at point 9 introduces heat energy into bolometers are used, each bolometcr consists of atlakc-v 2 material 1 by direct contact whereby at point 12 emitted form thermistor or sintercd semi-conductor material hav- `rays lll and 11 are characteristic ofthe thermal gradient s ing' a high negative coeicient of resistance which pro' 60 at the surface ofthe material duc to void 13. ,Detector vides a fast time response and high Sensitivity t0 very 3 'may bc similar to the detector illustrated in FIGURE I, small temper-awr: changes The radiantenefsv as sensed t but it is mbe understood that other thermal radiation by bolometers 17 and 18 is adapted to detect a thermal detectors thatprovide adiiiercntialtemperature measure-y gradient at the surface of material 1 in the following man. memmay also be' cmp10yd 1 ner, A chopper mirror 1 9, shown in detail'inFlGURE 65 M the. hemmt-cre dsn'be,1mmodg4 ff". a'pplyig' `2, ls rotated m the direction or arrow 20 by electromagi energy to he Surface of material l ris'mpormm that net 21- whlch .1S energized by con 22 connected. to a Sunil" an adequate surface temperature be achieved. `This ader able source ot electrical power. Armature 23 ls attached uate surface temperature-varies with the particular ma- I to one end, of sh'ft 24, the other end mountln cho er "q mirror Theaacon of O ang chopper rmfplg 70 terral being inspected and depth of lnspectlonbut by way lalternately impinges focussed rays 15 and 16 upon bolomlf example .5 appoximamy 45 C- fof bar s mck 0 Steel- -4 V-eters 17 (as shown) and 13. A black body reference 'W' cimmed molybdenum- A le'mpmmre dfkfemm Off" source v'is ypositioned adjacent chopper mirror 19 inavv i-04 C--IS readily delffledflhefby Permlllmg lhC HCC- manner l0 be hereinaflcr described in detail, A black' l tion of subsurface voxdsat least as small as 0.020 inch body may be described as a device which emits radiant 75 diameter. Tlledepth to which the voids may be detected. f

I to detect subsurface voids. FIGURE 5 represents the 25 comprising the desired thickness of coating S0 and hereinabove been described merely to illustrate my methutilizing a microscopic optical system providing a very," narrow 'eld of view and .the 4smallest bolometer flaite" v obtainable to achieve .a greater resolution and thereby' G0 radiant energy source is preferably attached to the side of the detector thus facilitating inspection of complex sur-v A swing over the work from 'a column support. such as av radial drill press andthe material being inspected remains Y that my invention makes available an improved method `and apparatus for determining the presence tud size ot' 75 is dependent on the intensity of the energy source, amore 'intense source permitting detection to a greater depth. i. v FIGURE 4 illustrates a test specimen 3 8 in which a number of ditferent diameter holes have. been drilled,

namclyv- These holes are in ell'ect voids. The test, 5 specimen 38 is rotated in the direction 44 by means of a.

' drill press machine, for example, and radiant energy is applied to the specimen surface in a manner as shown in FIGURE- l. FIGURE 5 indicates a graphical result oi` the test performed on the specimen in FIGURE 4 wherein m a suitable detector is ernployed,'as heretofore described n by the larger drilled hole 42. Thus the recorded results of FIGURE 5 clearly'indicate the sensitivity of my rnethotL A in recognizing voids'oi different size. 2Q

variation in the thickness of a heat conductive coating 50 on a base material 1 is'dctected, as illustrated inA FIGURE 6. in thc following manner. A reference body base material 1 is heated by splitting the radiation beam 25 8 that heats material 1 in .FIGURE l, thus providing uniform heating of the reference body 2 5 and material lbeing inspected. The reference body 25 may also comprise a portion of the inspected material wherein the coating thickness is known to be correct. 'the heated 30 material and reference body m ay be comparcdday the a0 detector and a diilerence in the times as indicatcd on r'e-` c order 31 is proportional .to the variation in the thicknessf., ofthe coating on, the inspected material .from-the rcllescnca'ww.

body. inthe alternative, :1s-illustrated inl FlGURE 7,'.

detector 3 may be positioned on the same side or' the in 45' specte'd material as the radiation sourceid, in which cas'.`

the times for emission of radiant energy from the coating Y surfaces are compared. In this alternative method, there may be a nezd to employ a radiation reflector on the bottom side of the base material, depending upon the' thickness of the base material. These methods permit detection of a 0.010 inch variation in thickness.l

Although a conventional thermal radiation detector has od for detecting voids'and other tlaws, it is to be under- '',5 stood that a preferred embodiment of the 'apparatus in accordance with my invention comprises a detector detect voids at leastas small as 0.020 inch diameter.'

faces and providing a more simple and cllicient device. It" v is to be further understood that the thermal radiation '65.-

detector and energy source may remain stationary and the ,1' -material to be inspected is moved in relation thereto in v. 1 a manner determined by the geometry of the nateri'al.j-`

in the alternative, the detector and ycucrgysource inay'l stationary. f t From the foregoing description it can be appreciated codecs subsurface voids, variationsincoating thicknesses and VA'radiation detector and energy source as being on the lopposite side of the material from the energy source, as

- by effecting negative thermal gradients which are detected may be made in the particular embodiment of my invenmeans for directing the constant level of thermal energy .v iucuus for detecting the emitted radiant energy and i tection of minute subsurface flaws in a heat conductive I :i pair of thermal radiation detectors,-

other flaws in a hcat lconductive material. Although FlGURES 1 and 3 indicate theA location-of the 'thermaly sume side of the material being inspected, it'isto be understood that this is not a requirement, as"ill ustr:.tcd in FIGURE 8 and will depend upon the particular application. ln applications wherein the detector is ou the illustrated in FIGURES 6 and 8 cold spots will occur in the presence of voids or flaws as hitherto described, ther Y in the same manner as hot spots or' positive-thermal' gradients. The sensitivity and accuracy of the apparatus may be further improved by exposing opposite surfaces of the material being inspected to the detector and energy source, This. technique permits an accurate determina.- tion of the depth of the void or llaw. Further, voidsmay4 be concentrated beneath the material surface in one area, thus inspection of more than one surface is preferred. Having described a new and improved method and apparatus for detecting voids and other aws within ai' heat conductive material it is believed obvious that :nod

ieations and variations of my invention are possible in Y light of the above teachings. Thus, other energy sources may bc substituted for those disclosed hereinabove to i generate the desired heat within the material being inspectcd. .lt is therefore, to be understood that changes tion described which are within the full intended scope of the .im cntion as defined hy the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is: 1. Apparatus for detecting minutesubsurface aws in'. a heat conductive material comprising f y :m energy source for generating a constant' level ofthermal energy,A -Y

, from said source to the surface of a material being linspected whereby the energy impinging upon said surface provides heating at said surface at a constant rise in tempcraturein the absence of a subsurface l' flaw therein, said energy inducing a thermal gradient' upon the surface of said material in the presence of 'a subsurface tlaw, said surface emitting radiant en- 'ergy characteristic of the 'surface temperature, and

transforming the detected energy into a recordable signal that indicates the presence and relative size of the subsurface ilaws within said material as 'a unction of a. detected thermal gradient atthe surface `fthereof` 2. Apparatus for noncoutacting and nondestructive de material movable with respect to the apparatus corn- 'a source -of energy forgenerating a constant level of thermal energy,. l *Q 1 r means for directing and focusing the constant ievel of thermal energy from said source upon a relatively i-j-f lsmall arca of the surface of a heat conductive ma;

' terial whereby the energy impinging upon thesurface i f'.- effects a constant risc in temperatureA at said Surface y in the absence of iubsurl'ace flaws, said energy inducing a thermal gradient upon said surface in the presence of a subsurface Haw, said surface emitting radiant energy characteristic of the surface tempera n optical system in communication with the radiant energy emitted by the surface, said optical system' having a narrow field' of view for transmitting the 7 radiant energy emitted b'y said surface to a pair or" '.thcrmal radiation detectors,

means for alternatelySimpinging-the radiant `energy transmitted by said optical system and the reference xradiant energy source upon each of said dctcctorsmnd means for transforming the output of said detectors into p a recordable signal that indicates the presence and relative size of subsurface flaws within'saidmateriaj surface thereof.l v i 3. The combinationsct forth in claim 2.whercin"` Y` said source of energy comprises a source of infrarcdv'b radiant energy,

prises an optical element. v. 4. The combination set forth in claim 2 wherein said means for directing and focusing the energy cornprises a tube member whe'efrom said emitted.

5. A method for detecting subsurface flaws in a heat 20 'f contluctiwe material comprising the steps of 1 t heating a surface of a moving heat conductive matcrial by directing a constant level of thermal energy v upon the surface thereof to produce a constant rate therein and to induce a thermal gradient thereon in the presence of a subsurface tiaw,

heated material, and

cordable signal that indicates the presence and relative size of subsurface Saws as a function of a detected thermal gradient. N 1 6. The method set forth in-claim S wherein the step of heating the surface of t. heat conductive material and the step of detecting emitted radiation are performed -on the same side of the heat conductive materiah 7. The method set forth in claim 5 wherein the step of-i heating the surface of a heat conductive material and,-

' on opposing sides of the heat conductive materialiv heating the surface of a heat conductive material comprises transferring energy to the surface by conduc- 9'. The method set forth in claim 5 wherein the step of heating the surface of a heat conductive material comprises transfcrring energy to the surface by induction) conductive material and for determining the depth and Y relative size of the flaws comprising the steps of heating a rst surface of heat conductive material by i f l Y directing a focused-radiant energy source having an 1 alternately focusing the collected radiation uponthe 5 output of constant thermal energy upon a tirst sub surface of the heat conductive material to induce a 55 constant rate ofheatingthcreon in the absence of subsurface fiaws therein and to induce a temperature... l gradient thereon in the presence of a subsurfacetlaw, moving the focused energy source in relation tother material. L- 1 1 detecting radiation emitted from the heated first surface' ofthe material," 1 y if transforming the detected emitted radiation into a'rstf t recordable signal indicating the presence of the .suhsurface aws, 1. 1 .g heating a second surface of the material by directing a focused radiant energy source having an o utputof.' constant thermal energy upon a second suria'ceof. i v'the heat conductive material toinducc a constant rate of heating thereon in the absenceof subsurface -aws and toinducea temperature gradientv thereonvv inthe presence ofa subsurface flaw, detecting radiation emitted from the heated second sur Y:faceofthematerial@v 'i' ,transfer-.ning second detcctcd emitted radiation into comparing the first and second:recordableisignals .j the iiaws.'. f as a function of a detected thermal gradient at the conductive material comprising the steps of output of constant thermal energy opone-surface said means for directing anu` focusing the energy comi said source ofenergy comprises a iiame impinging upon 35 said surface and having a small cross section, andv dame is A optically vieu-ing a very small area of the-heated sur,

focusing the collected radiation upon a rst infraredA alternately focusing the collected radiation upon the of heating thereon in the absence of subsurface aws 2f comparing the outputs of the first and second detectors,

recording thc compared outputs wbereby subsurface flaws are indicated by traces of thermal gradients detecting radiation emitted from the surface of 'the transforming the detected emitted-radiation into a rc- "0 12. 'A method for detecting variations in the thickness of`a heat conductive coating joined to a base material.- 35. comprising the steps of heating a heat conductive coating joined to a base ma terial by directing a first part of a split focused radiant the step of detecting emitted radiation are performed-0- -'-moving the coated material in relation to the focused I 8. The method set forth in'clairn 5 whcreinhe step'o' optically viewing a small area of a heated surface ofi'A the coated material to collect radiation emitted there focusing the collected radiation upon a rst ness of coating and. base material by directing a see- "5 10. A method for detecting subsurface flaws in a heat lirst detector, comparing th'c outputs of recording the compared outputs whereby variations in 1 magnitude of the recorded indication is proportional '.to the magnitude of the variation from the coating,

13. The method set forth in claim 12 whercinthe' step of heatinga heat conductive coating and the step of focus f" 'Y ing the collected radiation upon a rst infrared energy i detector are performed on the same side of the heat 14. The method set forth in claim 12 wherein the oi heating a heat conductive coating and the step of focusj a second 'recordable signal indicating-the 1:'rescrzce-Y of the subsurface aws indicated by the first recordable signal, and 1 "Y determine therefrom the. depth and relative size/o l1. A method for detecting subsurface aivs in n heating a'surfacc of heat conductive material by dh rectmg a focused radiant energy source having an,

of a heat conductive material to induce a constant' temperature rise at the surface in the absence of subsurface llaws and to induce a thermal gradient there-` lon in the prescnceof a subsurface flaw therein, moving the material in relation to the focused energy source,

face to collect radiation emitted therefrom,`

energy detector,

- second detector while simultaneously focusing lreference energy source upon the first detector,

and the magnitude of the recorded thermal gradient c I is proportional ro thesize of the subsurface flaw.

- energy source having an output of constant thermal energy upon the coating surface,

energy source,v J

energy detector, v heating a reference body comprising the desired thickond part of the split focused radiant energy source upon the coating surface of the reference body, focusing radiation emitted bythe heated reference body upon a second infrared energydetector,

second detector while simultaneously focusing the radiation emitted by the reference bodyupon the the thickness of the coating are indicated and the thickness on the reference body.

conductive coating..

ing the collected radiation upon' a first infrared energy detector are performed on opposing sides of coated base material.:

v-Rgiex'e'zrmes Cied by ivhevExmin 1 ji 'UNITED sAEs PATENTS Mmm A2504-83-55 v- Asthcimer Z55-83.3

' Y UNlTED STATES PATENTS 1,859,336 V 7732 De Forest.

2,260,186 A 10/41 IVICNUUL.V 2,264,968 12/41 De Forest. 2,340,150 1/44 Sornes. 2,541,437 '.7/51- Presenti.

' RA1L1 H G. NlLsoN, Primm Examiner. l:Mmes w LAWRENCE, Examiner. 

1. APPARATUS FOR DETECTING MINUTE SUBSURFACE FLAWS IN A HEAT CONDUCTIVE MATERIAL COMPRISING AN ENERGY SOURCE FOR GENERATING A CONSTANT LEVEL OF THERMAL ENERGY, MEANS FOR DIRECTING THE CONSTANT LEVEL OF THERMAL ENERGY FROM SAID SOURCE TO THE SURFACE OF A MATERIAL BEING INSPECTED WHEREBY THE ENERGY IMPINGING UPON SAID SURFACE PROVIDES HEATING AT SAID SURFACE AT A CONSTANT RISE IN TEMPERATURE IN THE ABSENCE OF A SUBSURFACE FLAW THEREIN, SAID ENERGY INDUCING A THERMAL GRADIENT UPON THE SURFACE OF SAID MATERIAL IN THE PRESENCE OF A SUBSURFACE FLAW, SAID SURFACE EMITTING RADIANT ENENERGY CHARACTERISTIC OF THE SURFACE TEMPERATURE, AND 